Small-Scale Features of Marine Sediments and Their Importance to the Study of Deposit-Feeding

Techniques currently in use by sedimentologists for the study of marine sedment microfabric are of limited use for understandmg the relationship between sediment organic matter and mineral grains. In this article it is shown that by combining standard histological protocols for fixation and dehydration with petrological protocols for resin embedding and thin sectioning, very fine details of the sediment structure can be seen. Because of the ubiquitous presence of the organic matrix, organicmineral aggregates are not seen in situ. Other features of the sediment of importance to deposit-feeders, such as the presence of intact chloroplasts, can be observed through the use of epifluorescence illumination, while partially crossed polarizers help to delimit the grain boundaries. It is suggested that if these procedures can be combined with histological staining techniques, it may be possible to determine the potential food value of sedment on a scale equivalent to that perceived by infaunal deposit-feeders.

Nutrient Uptake by a Deposit-Feeding Enteropneust: Nitrogenous Sources

We measured carbon, nitrogen, protein, bacterial and microalgal abundance, and mineral-specific surface area in sediments from the feeding zone of undisturbed Saccoglossus kowalewskyi, as well as in their fresh egesta. Comparison of results using surficial material 1 mm) and the top 3 mm of sediments indicated ingestion of surficial material by the enteropneusts. Assuming the surficial sediment as a food source results in apparent absorption efficiencies of 15% for TOC, 35% for TON, 60% for protein and 86% for microalgae. The C:N ratio of the apparently absorbed material was 4.2, consistent with an amino acid-rich diet. Protein- nitrogen uptake, however, accounted for only about 28% of total nitrogen absorption, indicating a dominant use of non-protein nitrogen . Bacterial and microalgal contributions to dietary nitrogen uptake were no more than 3% and 4% respectively. Active worms maintain 2 foraging areas with an average total foraging volume of 0.9 cm3 and a volume ingestion rate of 0.06 to 0.12 cm3 ind.-1 h-1. If the preferred feeding zone of these enteropneusts is the nitrogen -enriched surficial layer, we estimate that their feeding activities will deplete the available food resources every 8 to 16 h and they may rely on biological and tidal redistribution of surface material.